This invention relates to spacecraft attitude control systems, and more particularly to unified spacecraft attitude control systems in which the thrusters need not be provided in distinct pairs about the center of mass, but in which the control processor accommodates the actual thruster orientations.
Many prior art attitude control systems require that the thrusters be provided in pairs, with the elements of each pair having the same moment arm about the center of mass or gravity of the spacecraft. When a spacecraft is being assembled, the location of the center of mass can move about within the spacecraft from time to time as a result of design changes in the bus or each payload. It is costly and inconvenient to change the thruster mounting arrangements each time the center of mass changes, especially considering that each spacecraft may have sixteen or more thrusters, and also considering that impingement of the thrusters plumes on spacecraft structures must be considered when orienting each thruster. It is desirable to accommodate such changes in the center of mass without rewriting software or changing the physical mounting of the thrusters. This was done in the prior art by rewriting the software after each change. Software changes might also be required in the prior art as a result of differences in the specific impulse characteristics of the various thrusters, or changes thereto.
A spacecraft attitude control system is described in U.S. Pat. No. 5,130,931, issued Jul. 14, 1992 in the name of Paluszek et al. The Paluszek et al system is directed toward the problem of rewriting attitude control software as a result of changes of the center of mass of the spacecraft which may occur from time to time during its design, or which may occur during operation. The Paluszek et al controller sums six difference equations, each of which represents the maximum force or torque which each thruster can produce and a throttling factor, to produce a single scalar equation relating the variables to a performance index. The performance index is maximized by an iterative process aboard the spacecraft to determine the throttle values to be applied to the thrusters. The iterative process is repeated each control cycle.
U.S. Pat. No. 5,140,525, issued Aug. 18, 1992 in the name of Shankar et al, describes an attitude control system for a spacecraft which reduces the on-board signal processing load required to calculate the thrust desired for each thruster. This is accomplished by loading a weighted pseudo inverse of a pulse-width-to-torque transformation matrix C onto on-board memory. This avoids the need for the flight computer to perform iterative calculations. However, the Shankar et al system is most advantageous when the thrusters are arranged in complementary pairs.